Related Stories

A massive molecular cloud in our galatic neighbourhood is pulsating in a way that is puzzling U.S. astronomers, who said the phenomenon has never seen in molecular clouds before.

Dr Charles Lada of the Harvard Smithsonian Centre for Astrophysics in Boston and colleagues studied the dark molecular cloud known as Barnard 68 and found the signatures of both in-falling and out-flowing material at different locations across the face of the cloud.

The team described the observations as pulsing, or beating like a heart, every 250,000 years. A paper describing the find was published yesterday in The Astrophysical Journal.

Lada's group were studying Barnard 68, with the IRAM 30-metre radiotelescope in Spain to find out if it was rotating, expanding or contracting. The cloud appears to be in a temporary state of near-equilibrium: Barnard 68's mass causes gravity, pulling inwards, but its temperature balances this by pushing outwards.

However, Australian astronomers were not overly excited by the discovery. "Wobbling is probably a better description than pulsing," said Dr Charles Tinney from the Anglo Australian Observatory in Sydney.

Molecular clouds are natural in the interstellar medium. Like our atmosphere, they are made up of a complex mixture of atoms. "They are a chance region of space, where there is more stuff per unit volume than other regions of space," Tinney explained to ABC Science Online.

Barnard 68 is a molecular cloud near the constellation Ophiuchus. Dark molecular clouds have a high concentration of dust and molecular gas which absorb almost all the visible light emitted from background stars, making them appear black in the centre.

The American astronomers believe that this particular cloud, together with a smattering of others nearby, are the remnants of a much larger cloud that was broken apart by the strong stellar winds and ultraviolet radiation of young and heavy stars as well as supernova explosions in the galactic neighbourhood.

Shockwaves from a nearby supernova have been put up as one reason for the 'wobbling', and Lada speculates that such a shockwave may have smacked into the molecular cloud in the relatively recent past. The blast from a supernova could easily provide the impulse that started these pulsations.

This theory is supported by other observations showing that Barnard 68 is located inside a hot 'bubble' within the interstellar medium - a rarefied zone cleared out by a supernova. But Tinney was not so sure. "Such observations could be interpreted in a number of ways," he said.

Barnard 68 is a typical example of small, dark molecular clouds known as Bok Globules, and is relatively close at about 300 light-years away. The cloud is enormous, about 24,000 times the average distance between the Earth and Sun. If our Sun were placed at the centre of Barnard 68, the cloud would extend out to 300 times the orbit of Pluto.

Astronomers do not know exactly how molecular clouds like Barnard 68 form, but molecular clouds are often stellar nurseries, in which young stars form from collapsing molecular clouds. Barnard 68, however, is a stable cloud containing no newborn stars.

Barnard 68 holds as much material about 1.5 times the mass of our Sun, but chilled to a temperature of -262°C, or only a few degrees above absolute zero, the lowest temperature that can be reached. It is therefore one of the coldest objects in the universe.